1. Field of the Disclosure
The disclosure relates generally to liquid flow actuation and control techniques and, more particularly, to the use of variations in surface tension in such techniques.
2. Brief Description of Related Technology
Microfluidic actuation techniques have been actively researched for use in biochemical assays and other applications. Droplet actuation techniques have generally relied upon surface forces (i.e., surface energy gradients), and have typically been implemented using electrowetting, temperature gradient, chemical surface gradient, and dielectrophoresis techniques. For instance, the surface energy gradients can be created by adjusting the relative degree to which an underlying surface is hydrophilic or hydrophobic. Surface forces can create flow in continuous films as well, using electrochemically generated surfactants, but at the expense of liquid contamination.
Other past work has studied Marangoni flows generated by contact heating a liquid film from below, causing hexagonal flow cells to be created in a spatially periodic manner. See, for example, Getling et al., “Cellular Flow Patterns and their Evolutionary Scenarios in Three-dimensional Rayleigh-Benard Convection,” Phys. Rev. E., vol. 67, pp. 46313/1-46313/4 (2003).
Generally speaking, past surface force-based and other microfluidic techniques have utilized patterned substrates or prefabricated microchips. Such microchips are often assay- or application-specific, which may restrict the use or scope of the techniques. Moreover, microchips, as prefabricated structures, generally lack the capability to be reconfigured.
Oil is being increasingly used as a liquid phase in biological and chemical analysis systems. Microdroplets of water emulsified in oil have been used as micro-scale chemical reactors in several applications including the concentration of dissolved solutes and nanoparticles, as well as the amplification of single DNA molecules. The low evaporation rates of oil make it a desirable collection medium for long-term non-toxic sampling of airborne bioparticulates, and its optical transparency has made it popular as a liquid medium for micro-droplet based embryo culture.
Within the context of microfluidic systems, prior work has focused on how to generate microdroplets of water within a continuous oil phase and manipulate them (one at a time) using electrophoretic or optical forces. For example, lasers and electrostatic probes have been proposed as tools for droplet manipulation. See, for example, Ashkin, “Application of Radiation Pressure,” Science, vol. 210, pp. 1081-1087 (1980).
Recent work has also been directed to flow manipulation in water. See Basu et al., “High Speed Microfluidic Doublet Flow in Open Pools Driven by Non-Contact Micromachined Thermal Sources,” Proc. Intl. Conf. on Micro Electro Mechanical Sys., Miami Beach, Fla., pp. 666-669 (February 2005). In this work, a micro-scale heat source was suspended above water to generate a high-speed doublet pattern.